This invention relates generally to a sidelobe canceller system for cancelling interference (i.e., jamming) signals from signals received by a main radar antenna. More specifically, the invention relates to a sidelobe canceller system for cancelling both barrage jamming and false-target-repeater jamming from a radar receiver by averaging a large number of samples of signals for preprocessing the interference and cancelling barrage jamming, and then averaging a small number of samples for removing false-target jamming.
Barrage jammers transmit continuous interference over a frequency band which is broad enough to include the frequency band of a radar. Such interference may appear over substantially all of the radar display.
False-target-repeater jammers receive signals from the sidelobes of a radar antenna, delay and amplify the signals, and return the signals with enough gain to be received through the sidelobes so that false or apparent targets appear on the radar display.
The response time of conventional analog cancellers is too slow, that is, the operational bandwidth of such cancellers is too low, for cancelling false targets. Therefore, the most typical method for cancelling both forms of jamming includes an analog canceller system for cancelling barrage jamming and a sidelobe blanker system for removing false targets. The disadvantage of this method is that a sidelobe blanker responds to barrage jamming as well as to false targets. Therefore, if the radar is being jammed by barrage and false target interference, the sidelobe blanker will blank not only the false targets but also the barrage jamming, thereby possibly continuously blanking the entire radar display. Thus, in the presence of barrage interference the sidelobe blanker must be turned off and false targets are not blanked. Some sidelobe blanker systems are designed to shut off so as not to continuously blank the display, but such systems do not blank false targets while in a shut-off position.
Existing digital canceller systems cancel barrage jamming. Such systems require a number of cancellers in a main signal channel and in each of M auxiliary signal channels for cancelling interference from M barrage jammers. Each main-channel canceller must average a large number n (n&gt;&gt;2) of samples of main and auxiliary signals, that is, long-time averaging, in order to minimize any cancellation of the desirable component of the main signal at the main-channel canceller. For example, if a main-channel canceller averages over only 2 or 3 samples, that canceller will cancel 1/2 or 1/3, respectively, of the desirable signal which is fed to the canceller. However, cancellation of false targets requires a fast response time, or short-time averaging, that is, averaging over a small number (approximately, 2-4) of samples. If each main-channel canceller of an existing digital canceller system performs short-time averaging, both barrage jamming and false targets may be cancelled but, as previously indicated, a significant amount of the desirable signal will also be cancelled. This results in an unsatisfactory output of the desirable signal.